The present invention relates to a control technology of a variable valve timing mechanism constituted to variably control valve timing of an intake valve and an exhaust valve by changing a rotation phase of a camshaft relative to a crankshaft using an electromagnetic brake.
There has been known a conventional variable valve timing apparatus for an engine, for changing a rotation phase of a camshaft relative to a crankshaft by controlling a rotation delay of the camshaft relative to the crankshaft based on a friction braking by an electromagnetic brake, to variably control valve timing of intake and exhaust valves of an engine (Japanese Unexamined Patent Publication 10-153104).
In this variable valve timing apparatus, for example, a basic control amount of the electromagnetic brake is calculated based on a target rotation phase (target angle) and an engine rotation speed, while calculating a feedback control amount from a deviation between the target rotation phase and an actual rotation phase. Then, a final control amount (for example, duty control amount) is determined from the basic control amount and the feedback control amount, to control a current flowing in an electromagnetic coil constituting the electromagnetic brake.
The variable valve timing mechanism according to the above electromagnetic brake control has high response characteristics compared with a variable valve timing mechanism according to an oil pressure control. Therefore, when a change in target angle is large, an actual angle (rotation phase) is abruptly changed, resulting in a possibility that an abrupt change occurs in drivability to bring an engine stall.
The present invention has been achieved in view of the foregoing problems, and has an object of suppressing an abrupt change of valve timing, ensuring stable drivability, and preventing an occurrence of engine stall, in a variable valve timing mechanism according to an electromagnetic brake control.
In order to achieve the above object, the present invention is constituted to control a variable valve timing mechanism that variably controls valve timing of an intake valve or an exhaust valve by changing a rotation phase of a camshaft relative to a crankshaft due to friction braking of an electromagnetic brake, while limiting a change rate of the rotation phase.
According to the present invention, even when a target value of the rotation phase is largely changed, since the change rate of the rotation phase to be actually controlled is limited, an abrupt change of valve timing can be suppressed, to thereby prevent an occurrence of engine stall.
The other objects and features of this invention will become understood from the following description with accompanying drawings.